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September 11, 2012

Predicting Waves To Boost Marine-based Energy

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April Flowers for redOrbit.com - Your Universe Online

There is a great search going on for alternative sources of energy, to wean our society off of fast-disappearing fossil fuels. So far, most scientists have focused on sun and wind energy because they are so easy to harness.

There is potential for wave energy from the ocean, but harnessing it is more problematic, presenting some pretty specific challenges that make it a less promising resource.

Because wave size and force varies so greatly, it is very difficult to tune Wave Energy Converters (WECs) to harvest the maximum amount of energy from waves. This leads to intermittent energy collection. WECs also have to withstand the elements, harsh winds and storms, all of which are capable of destroying the devices.

Researchers from Tel Aviv University's School of Electrical Engineering and Center for Renewable Energy, along with colleagues from the University of Exeter, have developed a control algorithm that, when used with previous wave-prediction technology, helps WECs calculate the correct amount of force needed to collect the maximum energy possible. This allows the device to respond to each wave individually. The system, which was recently reported on in the journal Renewable Energy, doubles the energy previously collected by WECs.

Professor George Weiss of TAU explains that WECs have two parts — a fixed or weighted lower part, possibly attached to the ocean floor, and an upper section that moves up and down based on the motion of the water. The resistance force between the parts generates energy, but unlike wind turbines or solar panels that collect as much energy as comes their way, WECs need to adjust themselves to each oncoming wave to function properly. This requires knowledge of the characteristics of the oncoming waves.

If there is no resistance between the two parts, the upper moves freely and no energy is collected. Likewise, if there is so much resistance that it suppresses movement, the device turns rigid and no energy is collected. The ideal is a happy medium base on measurements of the incoming waves.

Prof. Weiss and his team developed a control algorithm that is responsible for setting the correct resistance force for the WEC based on the predicted wave information. A processor attached to the WEC runs the algorithm five times per second in order to determine and then implement an optimal mechanical response to the coming waves.

Simulations using wave data gathered from the ocean have been run in the laboratory. Combining existing wave prediction technology with the new algorithm improved energy collection by 100 percent — doubling the amount of energy that WECs previously collected.

The most important element, according to Weiss, is the height of the wave. The WEC needs to know the height in advance in order to prepare.

"You would think that the longer the WEC knows the wave height in advance, the better, but in a surprising finding, it turns out that a one-second prediction horizon is enough," he says, noting that a longer prediction time does not actually improve the energy harvest.

Their findings could not only help to improve the functioning of the WECs that are already in use in places such as the East Coast of the US and the Atlantic Coast of Spain, but could help the technology become more competitive. Currently, marine energy is fifty times more expensive to collect than the market price for the energy itself – as solar and wind energy were in their infancy, says Prof. Weiss. But with the improvement of WEC structure, performance, and mass production, it could become commercially viable.